The discovery of the syenogranite with an age of 126 Ma in the depth of Tongling: Evidence from 3000 meters scientific drilling

Tongling Scientific Drilling revealed that there exists a red syenogranite at 1775 meters below the surface in Shujiadian,Tongling,which has never been seen at the surface or in other drill holes of Tongling area before,and drilling about 700 meters still intersects the rock. Studies show that the syenogranite is characterized by higher content of SiO₂ and Na₂O+K₂O, and lower content of MgO, Ba and Sr, enrichment of HFSE (high field strength element) such as Zr, Nb, Y and LREE (light rare earth element) except Eu. The rock has a ratio of 10000Ga/Al>2.6,the total Zr+Nb+Ce+Y>350×10^-6, obvious Eu negative anomaly in the chondritenormalized REE patterns and depletion of Ba, Sr, P and Ti in the primitive mantle-normalized trace element diagram, showing Atype granite property. Zircon LA-MC-ICP-MS dating yielded a U-Pb age of 126 Ma for the A-type granite, which is consistent with the age of the other A-type granites in the middle and lower reaches of the Yangtze River (MLRYR), and belongs to the product of the second episode in the three episodes of the magmatism in the area. Researches indicate that the three episodes of magmatism in the MLRYR region had a closed relation to the multiple lithosphere delamination. With the continuing delamination, the deep magma chamber near the crust-mantle migrated to the shallow part, causing partial melting of the crust to form A-type granite when the magma was emplaced along the deep fault. It also marked a peak pull-apart in the tectonic background of the MLRYR region.